To produce uniformly strong, metallurgically-acceptable welds, a spot welder must be operated in a way that takes into account variations that exist in weld sites. Such variations exist because wire and the parts to which they are to be welded vary in thickness, surface finish cleanliness and purity. All of these mechanical variations affect the magnitude of the electrical resistance at the weld site. A typical range of site-to-site resistance variation is such that weld sites having a nominal or average resistance of about 15 milliohms are subject to a normal variation to a low of about 3 milliohms to a high of about 30 milliohms. This constitutes a percentage variation of +100% from the nominal to the high and of -80% from the nominal to the low. Such variations in the electrical resistance from one weld site to another will cause variations in the magnitude of instantaneous electrical power delivered to the weld sites if the same amount of instantaneous drive current is induced in the weld sites. Producing a strong, metallurgically-acceptable weld depends upon adhering to a weld schedule having limits involving time and power so as to deliver a right amount of energy within a right amount of time. If the amount of power being delivered is outside limits of the weld schedule a strong, metallurgically acceptable weld will not be produced.
Generally, spot welders include adjustment controls to enable an operator to select a desired setting for the magnitude and duration of the current that the spot welder will induce in a weld site. Thus, if the magnitude of the resistance at a weld site is ascertained in advance, it is possible for the operator to set the adjustment controls of the spot welder appropriately for the weld site. However, in a production setting, it is undesirable to take the time required to use the traditional method to measure the weld resistance.
The traditional method of measuring weld resistance is to pass a known d.c. test current through the series path defined by the welding electrodes of the spot welder and the weld site, and measure the resulting voltage that is developed across the welding electrodes. It is desirable to include the welding electrodes in this series path so that any effects caused by variations in contact area and pressure exerted by the welding electrodes is taken into account in the measurement. However, the welding electrodes are connected within the spot welder to the output drive circuitry and in almost all spot welders such output drive circuitry includes a transformer secondary winding, the opposite ends of which are connected to a respective one of the welding electrodes. If steps are not taken to disconnect the output winding during the measurement operation, the secondary winding would be in parallel with the resistance of the weld site, and the low resistance of the secondary winding would shunt the majority of the d.c. test current. Taking the time to disconnect the output winding is highly undesirable in a production setting. Further, there may be other reasons making it undesirable to disconnect circuitry from the transformer. For example, the spot welder drive circuitry typically is designed such that its proper operation depends in part on the load impedance it drives, and disconnecting such drive circuitry from the transformer can adversely affect the operation of such drive circuitry.